Typical repair cost ranges
Tankless repair cost is bimodal — most jobs are cheap, a few are catastrophic. Service call to diagnose: $125–$250 (most plumbers credit this toward repair if you proceed). Common repair labor: 1–2 hours at $100–$180/hour.
| Issue | Part cost | Labor | Total |
|---|---|---|---|
| Flow sensor replacement | $60–$140 | $100–$180 | $160–$320 |
| Igniter replacement | $80–$180 | $120–$220 | $200–$400 |
| Flame rod / sensor cleaning | $0 (cleaning only) | $100–$160 | $100–$160 |
| Gas valve replacement | $200–$450 | $180–$280 | $380–$730 |
| Combustion fan replacement | $180–$380 | $160–$260 | $340–$640 |
| Control board (PCB) replacement | $280–$650 | $140–$220 | $420–$870 |
| Descaling (annual maintenance) | $30 (vinegar) or $80 (chemical) | $180–$280 | $210–$360 |
| Heat exchanger replacement | $650–$1,400 | $400–$700 | $1,050–$2,100 |
| Venting repair or replacement | $120–$450 | $180–$400 | $300–$850 |
The deciding question for older units (10+ years) is heat-exchanger health. Heat exchanger replacement on a 10-year-old unit is often the moment to replace the whole unit instead — $1,500 in heat exchanger + labor versus $4,000 for a new unit with a fresh 15-year warranty. The repair makes sense if other components are also fresh; the replacement makes sense if the unit has had multiple service calls in the last 18 months.
The five problems behind 80% of service calls
1. Scale buildup in the heat exchanger
By far the most common cause of tankless failure or underperformance. Hard water (above 7 grains per gallon) deposits calcium carbonate on the inside of the heat exchanger tubes. Scale acts as insulation between the burner and the water, reducing efficiency and triggering high-temperature errors. In severe cases the exchanger develops hot spots that lead to pinhole leaks.
Diagnostic clues: error codes related to overheating (E10, E11, E12 on Rinnai; E351, E352 on Navien), reduced GPM at the same temperature setting, or audible knocking from the unit during high demand. The fix is descaling — circulating vinegar or a commercial descaler through the heat exchanger for 45–60 minutes via the service valves.
If service valves were never installed (some early or budget installs skipped them), the descaling requires removing the unit from the wall — a $400–$700 labor premium. Worth getting service valves added at first service if missing.
2. Flow sensor failure
The flow sensor tells the unit that water is moving through it, which triggers the burner to fire. When the sensor fails, the unit either does not fire at all (cold water out of the hot tap), fires intermittently (cold/hot/cold cycling), or fires unnecessarily (small hot-water cycling from a leaky faucet).
Diagnostic clues: error codes related to flow detection (E1 on most Rinnai, E422 on Navien), or hot water that comes only after running the tap for 30+ seconds. Replacement is straightforward — the sensor sits in the cold inlet, costs $60–$140, and takes a plumber 30–45 minutes to swap. A user-replaceable fix on most units if you are mechanically inclined.
3. Igniter or flame rod problems
Tankless heaters use a hot-surface igniter or spark igniter to light the burner each cycle. Igniters fail over time — typical life 8–12 years. When the igniter fails, the unit clicks but does not light.
The flame rod (a separate small probe that confirms ignition occurred) gets covered in carbon deposits over time and stops detecting flame, causing the unit to shut down on a "flame loss" error. Cleaning the flame rod with fine-grit sandpaper restores function — a $0 fix that plumbers charge $100–$160 to perform.
Diagnostic clues: error codes E2, E3 on Rinnai; E003, E004 on Navien. Ignition click but no flame. Flame for a few seconds then shutdown.
4. Combustion fan / vent obstruction
The combustion fan pulls air for the burner and pushes flue gases out. Failures: bearings wear out (audible whining or grinding); a bird or wasp nest blocks the vent termination; a snowdrift covers the side-wall vent in winter.
Diagnostic clues: vent-related error codes (E14, E16 on Rinnai; E110, E111 on Navien). Unit fires briefly then shuts down. Strange noises from the unit during operation.
Field check: look at the exterior vent termination first. Many "broken tankless" service calls are actually a yellowjacket nest in the vent cap or a snowdrift packing the exhaust. Both are $0 fixes that homeowners can do themselves.
5. Control board (PCB) failure
The least common but most expensive routine repair. The PCB controls everything — flow detection, ignition timing, modulation, fault codes, the user interface. When it fails (typically year 8–12), the unit either does not respond at all or throws nonsensical fault codes that change between cycles.
Diagnostic clues: display flickers or shows random characters, unit refuses to turn on, error codes change every cycle, or the unit ignores temperature setpoint changes. PCB replacement costs $420–$870 installed. On a 10+ year unit, this is the decision point for repair vs replace.
What you can DIY before calling
Free DIY diagnostics that resolve roughly 25% of "broken tankless" calls:
- Check the vent termination. Walk outside, look at the exhaust outlet. Wasp nest? Bird droppings? Snow? Clear it.
- Check the air intake. Same termination on direct-vent units, separate intake on dual-vent units. Same check.
- Check the gas supply. Is the gas valve at the meter open? Are other gas appliances working? If only the tankless is dead but the range works, the issue is downstream of the meter.
- Reset the unit. Power off at the breaker for 30 seconds. Power back on. About 10% of fault codes clear themselves on reset.
- Clean the flame rod. Requires removing the front cover. The flame rod is a small ceramic probe on the burner — a 5-second cleaning with fine sandpaper resolves most "flame loss" errors.
- Clean the inlet filter. A small mesh strainer in the cold-water inlet. Clogged filter mimics flow sensor failure. Unscrew it, rinse, reinstall.
If none of the above fixes it: call a plumber. Tankless service is specialty work — your general HVAC tech may not have worked on one. Ask specifically: "Have you serviced a Rinnai/Navien/Rheem [model] before?" Familiarity matters more than years of experience.
Repair vs replace decision
Replace when:
- Heat exchanger has failed and unit is 8+ years old.
- Two or more major repairs in the last 18 months.
- Total cost of needed repairs exceeds 60% of the new unit price.
- Unit is no longer manufactured and parts are scarce.
Repair when:
- Unit is under 7 years old.
- Single-component failure (sensor, igniter, fan).
- Repair cost is under 35% of new unit price.
- Unit still has warranty coverage on the heat exchanger.
Most premium tankless heaters (Rinnai, Navien) carry 15-year heat-exchanger warranties. If your unit is under warranty and the exchanger fails, the part is free — only labor applies. Always check warranty status before agreeing to expensive repairs.
Brand-specific service notes
Rinnai: Largest US service network. Most plumbers have worked on Rinnai. Parts available through Ferguson and other plumbing wholesalers in 1–2 days. Diagnostic flow is unusually clean — the fault code table directly maps to the failed component. Annual descaling is the only required maintenance.
Navien: Smaller service network than Rinnai but growing fast. The ComfortFlow recirculation pump is the most common service component on NPE-A series — typical service at year 8–10. Heat exchanger is fully stainless so scale issues are less severe than competitor units.
Rheem (RTGH series): Service through Rheem's plumber network. Parts available through Home Depot's pro counter and via online Rheem parts vendors. Generally robust but the gas valve assembly is a known wear item around year 7–8.
Bosch / Bosch Therm: Limited US service. If your installer was the dealer, service is easy. If they have moved on, finding a qualified tech is harder than competitor brands.
Takagi: Owned by Rheem since 2009. Service through Rheem network. Same notes as Rheem.